This invention relates to the production of alpha, beta-unsaturated methyl esters by reaction between dimethylformal and carboxylic acid compounds of the formula RCH.sub.2 COOR' wherein R is a member of the class consisting of --H, -alkyl, -aryl, -aralkyl, -cycloalkyl, and -alkylaryl radicals. Where R is not hydrogen, the number of carbon atoms in R is preferably from 1 to 18. R' is selected from the group consisting of --H and --CH.sub.3 radicals.
It is well-known that methyl alpha-methacrylate and methyl acrylate can be prepared by reacting formaldehyde with a suitable reactant, i.e., methyl propionate and methyl acetate, in the presence of a suitable catalyst. This invention is directed to an in situ process for synthesis of alpha, beta-unsaturated methyl esters, e.g., methyl alpha-methacrylate (and methyl acrylate) from propionic acid (or acetic acid) and methyl esters of such acids dimethylformal, i.e., formaldehyde in the form of its dimethyl acetal of the formula CH.sub.3 OCH.sub.2 OCH.sub.3. The process requires the presence of a catalyst comprising a borosilicate crystalline molecular sieve, designated as AMS-1B, having the following composition in terms of mole ratios of oxides: EQU 0.9 .+-.0.2M.sub.2/n O:B.sub.2 O.sub.3 :YSiO.sub.2 :ZH.sub.2 O
wherein M is at least one cation, n is the valence of the cation, Y is a value within the range of 4 to about 600, and Z is a value within the range of 0 to about 160, and providing a specific X-ray diffraction pattern.
Unsaturated acids, such as methacrylic and acrylic acids, acrylonitrile and the esters of such acids, such as methyl alpha-methacrylate, are widely used for the production of corresponding polymers, resins and the like. Various process and catalysts have been proposed for the conversion of alkanoic acids, such as propionic acid, and various forms of formaldehyde to the corresponding unsaturated monocarboxylic acids, e.g., methacrylic acid, by an aldol-type reaction. Generally, the reaction of the acid and formaldehyde takes place in the vapor or gas phase while in the presence of a basic or acidic catalyst.
Various catalysts have been proposed for such reactions. For example, Vitcha, et al., I&EC Product Research and Development, 5, No. 1 (March, 1966) pp. 50-53, propose a vapor phase reaction of acetic acid and fomaldehyde employing catalysts comprising alkali and alkaline earth metal aluminosilicates, silica gel, alumina and the like. U.S. Pat. No. 2,734,074 teaches the preparation of acrylic ester by formaldehyde condensation with a lower alkyl ester in the presence of a dehydration catalyst comprising lead acetate suspended on silica gel. U.S. Pat. No. 2,821,543 teaches a similar preparation using basic metal compounds such as basic reacting salts or oxides of metals, i.e., manganese oxide, deposited upon a suitable carrier such as activated alumina or activated silica. U.S. Pat. No. 3,051,747 describes the preparation of acrylic acids by reacting an alkanoic acid and formaldehyde in the presence of a catalyst comprising an alkali metal salt of the alkanoic acid supported on alumina. The same reaction is also promoted by catalysts which include alkali metal or alkaline earth metal aluminosilicates, silica gel or alumina. Catalysts of this kind are described in U.S. Pat. No. 3,247,248 which teaches a process for the reaction of formaldehyde and acetic acid or propionic acid in the presence of a natural or synthetic aluminosilicate catalyst that may include alkali or alkaline earth metals, such as the aluminosilicates of sodium, potassium, rubidium, magnesium, calcium, strontium or barium. In addition, the use of silica gel in combination with an alkali metal or alkaline earth metal hydroxide as a catalyst for the reaction is described. U.S. Pat. No. 3,933,888 teaches the preparation of unsaturated acids, the esters and nitriles of such unsaturated acids wherein alkanoic acids, esters of such acids and alkyl nitriles are reacted with formaldehyde in the presence of a basic catalyst comprising pyrogenic silica. The pyrogenic silica is taught as especially effective when treated with activating agents which provide basic sites on the pyrogenic silica catalyst support, such as organic bases, inorganic bases of Groups IA, IIA and IIIB of the Periodic Table, particularly the alkali metal hydroxides such as potassium hydroxide and cesium hydroxide. The addition of a compound of a metal as an activating agent is taught as increasing the effectiveness of the catalyst.
Other processes and catalysts have been proposed for the preparation of methacrylic acid and esters. U.S. Pat. No. 3,089,898 teaches a process and catalyst for preparation of methyl acrylate which comprises contacting vapor mixtures of methyl acetate and formaldehyde with aluminosilicate catalysts, particularly alkaline earth metal zeolites, alkali metal zeolites and zeolites of certain heavy metals such as manganese, cobalt, zinc, cadmium and lead. Aqueous and alcoholic sources of formaldehyde are taught as useful. U.S. Pat. No. 3,089,899 teaches preparation of methyl methacrylate which comprises contacting vapor mixtures of methyl propionate and formaldehyde with zeolite catalysts, particularly certain synthetic zeolites, especially the aluminosilicates of Group IIA of the Periodic Table, such as magnesium, calcium, strontium and barium aluminosilicates, and manganous aluminosilicates. Aqueous of alcoholic formaldehyde or anhydrous paraformaldehyde can be used. U.S. Pat. No. 3,089,900 teaches preparation of methyl methacrylate using a catalyst consisting of potassium hydroxide impregnated on silica gel. G.B. Pat. No. 1,107,234 teaches a similar process using potassium, rubidium or cesium hydroxide on silica gel as catalyst. U.S. Pat. No. 3,089,901 teaches use of alkali metal metaborates on silica gel and alkali metal tetraborates on silica gel as catalysts. U.S. Pat. No. 3,089,902 teaches alkali metal silicate on silica gel as catalyst.
Accordingly, a number of processes using basic metal catalysts have been taught heretofore. Other process using basic metal compounds on silica gel catalysts are taught in U.S. Pat. Nos. 3,100,795; 3,247,248; 3,534,087; 3,670,016; 3,840,587; 3,840,588. But, although an alkali-treated silica gel improves the activity of the formaldehyde with regard to the desired reaction, at the same time, as is well-known, formaldehyde has a tendency to undergo undesirable side reactions owing to its high reactivity in alkaline media.
Processes to minimize or to avoid the aforesaid undesirable side reactions which formaldehyde undergoes in alkaline media have been taught. U.S. Pat. No. 3,535,371 teaches use of a niobium oxide catalyst on alumina. U.S. Pat. No. 3,845,106 teaches use of an unmodified silica gel. G.B. Pat. No. 1,491,183 teaches use of methylal instead of formaldehyde with a metal oxide catalyst, preferably Al.sub.2 O.sub.3. U.S. Pat. No. 4,085,143 teaches use of a catalyst comprising silica gel and a salt or an oxide of a metal selected from the group consisting of tantalum, titanium, niobium, and zirconium with an acid anhydride and formaldehyde. Boric acid deposited an alumina is also taught as a catalyst. U.S. Pat. No. 4,118,588 teaches a process and catalyst for preparing methacrylic acid and methyl methacrylate which comprises reacting, respectively, propionic acid and methyl propionate with dimethoxymethane in the presence of catalysts based on phosphates and/or silicates of magnesium, aluminum, zirconium, thorium and/or titanium and in the presence of water. Boric acid and/or urea can also be present. Preferably, the catalysts are modified with alkali metal and/or alkaline earth metal carboxylates and/or alkali metal compounds and/or alkaline earth metal compounds which yield carboxylates under the reaction conditions. Suitable modifiers are the carboxylates, oxides and hydroxides of lithium, sodium, potassium, magnesium and calcium as well as those of beryllium, strontium, rubidium, cesium and barium.
However, the processes and catalysts taught heretofore suffer from disadvantages which are greatly minimized in the process of the present invention. For example, the processes as described in Vitcha, I&EC, op. cit. p. 50, are inferior to the present invented process in that conversion of formaldehyde is low when acid concentration is low. Vitcha indicates that as the ratio of acetic to formaldehyde decreases, the competitive reaction of formaldehyde with itself to form polymers predominates, to result in lower conversion and yield. Other examples can be cited. U.S. Pat. No. 3,051,747 indicates that the major product of the process is not an unsaturated compound but a symmetric ketone. The process described in U.S. Pat. No. 3,247,248 is also inferior to the process of the present invention. Yield percent based on formaldehyde taught by U.S. Pat. No. 3,247,248 with 5:1 ratios of acid to formaldehyde is between 20 and 40 percent.
Quite unexpectedly, it has been found that a catalyst comprising AMS-1B borosilicate crystalline molecular sieve having the following composition in terms of mole ratios of oxides: EQU 0.9.+-.0.2M.sub.2/n O:B.sub.2 O.sub.3 :YSiO.sub.2 :ZH.sub.2 O
wherein M is at least one cation, n is the valence of the cation, Y is a value within the range of 4 to about 600, and Z is a value within the range of 0 to about 160, and providing a specific X-ray diffraction pattern, performs in a much superior manner for the present process with respect to conversion and selectivity relative to conventional catalysts. Whereas previously taught catalyst formulations require a basic metal on silica or alumina substrates, the catalyst of the instant invented process is a borosilicate crystalline molecular sieve catalyst. Yield and selectivity are also improved over previously taught catalysts. The improved process has several unexpected results. Whereas previously taught processes result in low formaldehyde-based yields of methyl alpha-methacrylate or methyl acrylate when the ratio of acid to formaldehyde is low, such as 1:1, the preferred acid:dimethylformal ratio for the process of the present invention is 0.5:1 to 20:1, preferably 1:1, with consequent economic advantage. Also, in previously taught processes, substantial amounts of acid often are formed from the ester from ester-cleavage side reactions. Even when the reaction is carried out in the presence of excess alcohol, the formation of acid via cleavage is not easily suppressed.
The process and catalyst of the instant invention circumvent the ester-cleavage mechanism by utilizing an acid:dimethylformal mechanism. In preparation of methyl alpha-methacrylate, the major products of the reaction, methyl propionate and methyl methacrylate, are easily separated by conventional methods. The methyl propionate can be reached with formaldehyde under suitable process conditions to prepare methyl alpha-methacrylate, and, alternatively, along with unesterified propionic acid, is recycled to the reactor. In this way, methyl alpha-methacrylate is conveniently synthesized in situ directly from propionic acid and without the need for a separate esterification section within the overall process. In addition, the present invention is with the use of a dry derivative of formaldehyde, dimethylformal.
An object of the present invention is to provide a process for making unsaturated methyl esters from saturated carboxylic acids and dimethylformal. A further object is to provide a process for making methyl acrylate. A further object is to provide a process for making methyl alpha-methacrylate. Other objects will appear hereinafter.